Conventionally, ethylene homopolymers or ethylene copolymers with α-olefins have been produced using a so-called Ziegler-Natta catalyst system comprising a titanium compound or a vanadium compound, serving as a main catalyst, and an alkyl aluminum compound, serving as a cocatalyst. Although the Ziegler-Natta catalyst system is highly active in the polymerization of ethylene, it has heterogeneous active sites, leading to a polymer having a wide molecular weight distribution, and in particular, an ethylene copolymer with α-olefin having a non-uniform composition distribution.
Recently, the development of a metallocene catalyst system, comprising a metallocene compound of Group IV transition metal in the periodic table, such as titanium, zirconium, or hafnium, and a methylaluminoxane cocatalyst, has been realized. Since the metallocene catalyst system is a homogeneous catalyst having a single active site, it has a characteristic of being able to produce polyethylene having a narrower molecular weight distribution and a more uniform composition distribution than when using conventional Ziegler-Natta catalyst systems. For example, EP Laid-open Publication Nos. 320762 and 3726325 or Japanese Patent Laid-open Publication No. Sho. 63-092621 and Japanese Patent Laid-open Publication Nos. Hei. 02-84405 and 03-2347 disclose metallocene compounds, such as Cp2TiCl2, Cp2ZrCl2, Cp2ZrMeCl, Cp2ZrMe2, or ethylene(IndH4)2ZrCl2, activated with methylaluminoxane as a cocatalyst to polymerize ethylene at high catalystic activity, thereby making it possible to produce polyethylene having molecular weight distribution (Mw/Mn) of 1.5˜2.0.
However, it is difficult to obtain polymers having high molecular weights using the above catalyst system. Further, in the case where such a catalyst system is applied to solution polymerization at high temperatures of 140° C. or more, polymerization activity is drastically decreased, and β-dehydrogenation mainly occurs, and thus the catalyst system is known to be unsuitable for the production of high molecular weight polymers having a weight average molecular weight (Mw) of 100,000 or more.
Moreover, U.S. Pat. No. 5,084,534, granted to Exxon, discloses the production of a copolymer having a narrow molecular weight distribution of 1.8˜3.0 and a uniform composition distribution by homopolymerizing ethylene or copolymerizing ethylene with 1-hexene or 1-octene at 150˜200° C. using a (n-BuCp)2ZrCl2 catalyst and a methylaluminoxane cocatalyst. In addition, EP Nos. 0416815 and 0420436, granted to Dow, disclose a catalyst having a geometrically constrained structure in which an amide group is linked to a single cyclopentadiene ligand to form the shape of a ring. The disclosed catalyst is highly active in the homopolymerization of ethylene or the copolymerization of ethylene with α-olefin not only in slurry polymerization but also in solution polymerization, and thus it is possible to produce a high molecular weight polymer having excellent reactivity with the comonomer and uniform composition distribution. However, such a catalyst system is disadvantageous because the stability thereof is drastically decreased as the temperature increases under conditions of high-temperature solution polymerization at 140° C. or more, and compatibility with a paraffin hydrocarbon solvent, typically used in commercial processes, is poor.
Although U.S. Pat. No. 6,207,773 discloses a metallocene catalyst in which cyclopentadiene derivative and an electron donating substituent, both of which are bonded to a Group IV transition metal, are crosslinked through a silyl derivative substituted with a cyclic hydrocarbon, it does not show suitable property for high-temperature solution polymerization.